The present invention relates, in general, to semiconductor device fabrication and, more particularly, to etch processes used in the fabrication of semiconductor devices.
The fabrication of semiconductor devices and/or integrated circuits often requires removing certain materials from a semiconductor wafer while leaving other materials on the wafer. This can be accomplished in a selective etch process that uses an etchant having different etch rates with respect to different materials. To characterize the selective etch process, an etch selectivity is defined as the ratio of the etch rate of one material to the etch rate of another material. For example, an aqueous phosphoric acid solution having a concentration of approximately 85 percent heated to a temperature between 165 degrees Celsius (xc2x0 C.) and 185xc2x0 C. is routinely used for removing silicon nitride structures from a semiconductor wafer while leaving exposed silicon dioxide structures on the wafer. At the temperature of 165xc2x0 C., the phosphoric acid solution etches silicon nitride at a rate of approximately 6 nanometers per minute and etches silicon dioxide at a rate of no more than 0.25 nanometers per minute. The resulting etch selectivity is at least 24:1.
The etch selectivity of an etch process depends on the temperature, concentration, and composition of the etchant. Consequently, the etch selectivity usually changes as more wafers are processed in the etchant. For example, the nitride to oxide etch selectivity of the etch process using the phosphoric acid etchant is approximately 24:1 when the etchant is fresh. After processing approximately 1000 wafers having silicon nitride thereon, the etch selectivity increases dramatically to 50:1 or greater. This selectivity variation adversely affects the efficiency, reliability, and yield of the semiconductor device and/or integrated circuit fabrication processes.
Accordingly, it would be advantageous to have an etch process that has a stable etch selectivity and an apparatus for performing the etch process. It is desirable for the etch process to have a high etch selectivity. It would be of further advantage if the etch apparatus can be adapted from existing etch apparatuses.
A general object of the present invention is to provide an efficient and reliable etch process and an apparatus for performing the etch process. It is a further object of the present invention for the etch process to be capable of producing semiconductor devices and/or integrated circuits having high performance, high reliability, and high yield. Another object of the present invention is to implement the etch process with modifications to existing etch apparatuses.
These and other objects of the present invention are achieved by adjusting and controlling the composition of the etchant during the etch process. For example, a selective etch modifier can be introduced into the etchant. The selective etch modifier alters the etch rates of certain materials but has no significant effect on the etch rates of other materials, thereby modifying the etch selectivity of the etch process. By monitoring and controlling the concentration of the etch rate modifier in the etchant, a stable etch selectivity is maintained during the etch process. The etch rate modifier can be either a selective etch intensifier or a selective etch rate suppressor. The selective etch intensifier selectively increases the etch rate of certain materials. On the other hand, the selective etch rate suppressor selective decreases the etch rate of certain materials.
In a preferred embodiment of the present invention, a hot phosphoric acid solution is used as the etchant for etching the silicon nitride on a semiconductor wafer. A recirculating path is established for the hot phosphoric acid etchant. A high surface area structure such as, for example, a carbon matrix filter is coated with silicon nitride. The carbon matrix filter is installed in the recirculating path for the etchant. As the etchant in the recirculating path flows through the carbon matrix filter, it dissolves the silicon nitride coated on the carbon matrix filter. The dissolved silicon nitride significantly reduces the etch rate of silicon dioxide on the semiconductor wafer. The etch rate of the silicon nitride on the semiconductor wafer is substantially unaffected by the presence of the silicon nitride in the etchant. Therefore, the silicon nitride dissolved in the hot phosphoric acid etchant functions as an etch rate modifier that enhances the etch selectivity of the etch process. More particularly, the dissolved silicon nitride functions as a selective etch rate suppressor that substantially inhibits the etch of silicon dioxide on the semiconductor wafer. The concentration of silicon nitride in the etchant can be monitored and adjusted to maintain a stable etch selectivity of the etch process.